A. Field of the Invention
The present invention relates to a method useful for separating one gas from a mixture of gases. More particularly, the present invention relates to a method for producing in a highly effective and efficient procedure relatively pure nitrogen from air utilizing a plurality of serially arranged bundles of hollow fiber membranes.
B. Prior Art
Membranes have been employed which use the principle of selective permeation to separate mixtures of gases into various components. Each gas has a characteristic permeation rate that is a function of its ability to dissolve into and diffuse through a given membrane. It is also known to utilize a bundle of hollow fiber membranes positioned within an elongated shell for separating one or more gases from a mixture of gases by allowing gases to permeate selectively through the membrane. For example, it is known to make relatively pure nitrogen from air by moving air under pressure into one end of an elongated container filled with a plurality of juxtaposed axially hollow membrane fibers running longitudinally of the container. Oxygen, carbon dioxide, water and other gases will permeate through the membrane fibers; but nitrogen will permeate to a lesser extent. The gases passing through the membrane and separated from air are withdrawn from the downstream side of the membrane. As a result the portion of the air which does not permeate the membrane fibers after contact with the active membrane surface is relatively pure nitrogen. The nitrogen gas will inevitably have a small amount of oxygen. Normally, reducing the flow rates of the air through the container housing the hollow membrane fibers will result in greater separation of the unwanted gases of high permeability from nitrogen. Slowing down the air flow rates results in more separation but with a resulting expensive operational cost. The driving force in the gas separating membranes is the difference between a stream component's partial pressure on the upstream surface of the membrane and the partial pressure on the downstream surface of the membrane. Obviously, it would be desirable to provide a process where for a given flow rate and driving force, one is able to produce nitrogen even less contaminated with other gases such as oxygen, carbon dioxide and water vapor.
Using air as the gas source, membrane gas separators are commercially and economically employed to provide a relatively inert gas by separating most of the oxygen from the gaseous components of the air, thereby leaving mostly nitrogen. Unfortunately, the removal of the oxygen is not sufficiently complete for many purposes. However, it has been practical to reduce the oxygen content to about 0.5 to 5% by using membrane gas separators.
Gas membrane separators arranged in series have been disclosed for separating gases, such as for recovering hydrogen from mixtures of hydrogen and methane. The capturing of relatively pure nitrogen with levels of oxygen below 1000 ppm using serially arranged separators without the need of recycle loops or interstage compressors has not been known.
Series operation of membrane separator units has been previously used for gas separations but always where the higher pressure feed gas was on the external surfaces of hollow fibers and where series operation was employed principally to improve gas mixing and distribution efficiencies on the feed side of the hollow fiber membranes. Additionally, series operation of membrane separators has been previously employed in applications where it was beneficial to operate a first separator at a higher pressure than subsequent separators in the serial train of units. In these cases too, feed gases were contacting the external surfaces of the hollow fiber membranes.
There remains a need to produce from air an inert gas with much lower levels of oxygen at economically acceptable throughput rates. Inert gases having oxygen levels in the range of 0.1% (1,000 ppm) or less are required when the inert gas is used to purge or blanket certain chemical processes, analytical instruments, flammable materials and other specialized applications. Heretofore, the production of nitrogen with such low levels of oxygen at acceptable throughput rates has not been satisfactorily accomplished using membrane separators.
Inert gases containing less than 1,000 ppm oxygen are currently prepared for commercial use by alternative processes, such as pressure swing adsorption, catalytic converters, cryogenic separation and the like. There is a need to provide a more economical process for the generation of substantially pure nitrogen from air wherein the level of oxygen in the nitrogen is below levels of 1,000 ppm.